A wide area communication system is generally referred to as a communication system which provides coverage for communication units travelling in a substantially large area, such as a metropolitan area and neighboring suburbs. In a wide area communication system, the communication units usually comprise substantially high power mobile two-way radios which are installed in a vehicle and communicate with each other via wide area repeaters which include substantially high power transmitters to provide the necessary communication coverage.
An on site communication system generally refers to a communication system which provides communication coverage for portable communication units confined to substantially smaller local sites, such as a building. The communication units in an on site communication system usually comprise small and substantially low power portable radios generally having an output power range between 30 to 100 mWatt which provide prolonged battery life.
It is desired to extend the communication coverage of the low power portable communication units operating within a plurality of on site communication systems and to provide full duplex communication capability with the high power mobile communication units operating in the wide area communication system. However, the low power output of the portable communication units may inhibit communication of messages with the wide area repeater. That is, communication messages transmitted from the portable communication unit may not reach the wide area repeater due to a communication unit's significantly lower transmitter output power. It is also typical for an on site communications system to utilize a different communications protocol than that of a wide area communications system, rendering the on site communication units incompatible with use in the wide area system. Hence, a communications user of the prior art desiring communications in both an on site area and a wide area is required to carry at least two communication units. This invention addressees the problem of integration of wide area and on site communication systems, thereby making it possible for a single portable communication unit of acceptable complexity to be used for all communication needs.
The communication in both wide area and on site communication systems makes use of radio frequency (RF) communication channels for communicating the communication messages. Radio frequency (RF) communication systems that make use of time division multiplexing (TDM) are known. In such systems a communication frequency (or frequencies) is divided by time into frames and time slots, and a plurality of communication units are assigned time slots to support their communication needs. The communication units communicate their messages at their respective assigned time slots through a number of repeaters in conjunction with one central communication controller. In order to facilitate the channel assignment mechanism, such systems generally provide one or more dedicated control time slots that support the channel assignment and other control functions. An important advantage of a TDM radio communication system is the capability of providing full duplex communication between two communication units on a single RF communication channel.